The present invention relates generally to prevention of deadbands in optical communication systems. More specifically, the present invention relates to methods and apparatus for using optical devices without the creation of deadbands.
While fiber-optic cable is finding widespread use for data transmission and other telecommunication applications, the cost of new installed fiber-optic cable presents a barrier to increased carrying capacity. Wavelength division multiplexing (WDM) allows multiple signals at different wavelengths to be carried by a fiber-optic line or other waveguide. Presently preferred wavelength bands for fiber-optic transmission media include those centered at about 1300 nm (nanometers), about 1550 nm (C band), and about 1590 (L Band). The C band, with a useful bandwidth of approximately 10 to 40 nm depending on the application, is preferred in many applications because of its minimal absorption and the commercial availability of erbium doped amplifiers that operate in the C band. Ideally, to substantially increase an optical fiber""s signal carrying capacity, the C band or L band would be divided into multiple discrete channels through a technique referred to as dense wavelength division multiplexing. Dense wavelength division multiplexing can separate this bandwidth into multiple wavelengths, allowing up to 80 or more wavelengths. For example, the International Telephony Union (ITU) Grid provides standard center wavelengths for channels in the 1550 nm wavelength band, at 100 GHz spacing (approximately 0.8 nm).
In addition to the requirement for multiplexing multiple signal wavelengths onto a single optical fiber, the need exists to route one or more channels of the multiplexed channels to differing locations. This routing ability, known as add/drop functionality, is accomplished by dropping out (demultiplexing) xe2x80x9coldxe2x80x9d channels from the optical fiber and adding in (multiplexing) xe2x80x9cnewxe2x80x9d channels. One preferred method of multiplexing and demultiplexing optical wavelengths utilizes thin film optical filters to add and drop portions of the communications spectrum. In order to add in or drop out parts of the continuous spectrum of closely packed wavelengths, allowance must be made for deadbands, or transition regions, of thin film optical filters. In these regions, system designers must deactivate signal wavelengths as they are affected by the rising edge region and falling edge region of optical filters. Other optical devices or components may have deadbands (spectral regions which are effectively unuseable or which are not subject to the desired effect of the device) for other reasons.
As system designers strive for greater channel counts with increasing bandwidth, the need for fewer and smaller dead zones within the overall signal band increases. Systems designers typically compromise between this need for fewer dead zones and the width of the various wavelength bands being combined in the system. While add/drop features and functionality are becoming more important, adding add/drop capability can reduce available signal bandwidth through the creation of deadbands.
Additionally, optical amplifiers typically utilize thin film optical filters and thus also suffer the loss of bandwidth due to the deadbands of the thin film filters. An amplifier system combining the C and L bands of an erbium amplifier produces deadbands between these two bands of approximately 10 nm.
Accordingly, it would be highly advantageous to provide for the capability to add, drop, and amplify portions of the communication spectrum without substantially reducing signal bandwidth.
The present invention provides advantageous methods and apparatus for adding and/or dropping channels in an optical communication system without substantially reducing overall system bandwidth. According to one aspect of the present invention, an optical filter is used to drop a wavelength range from an optical signal. Prior to the optical signal entering the optical filter, one or more fiber Bragg gratings and an optical circulator are used to reflect a portion of the communication spectrum which would normally lie within the deadband region of the optical filter.
According to another aspect of the present invention, an optical filter is used to combine a first optical signal and a second optical signal to form a combined optical signal. Neither the first optical signal nor the second optical signal includes channels within a deadband region of the optical filter. One or more fiber Bragg gratings and an optical circulator are used to add a third optical signal to the combined optical signal. The third optical signal includes channels within the deadband region of the optical filter.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.